Ammonia is produced industrially by passing a mixture of gases containing a large proportion of hydrogen and nitrogen, essentially in a 3-to-1 ratio, over a catalyst in an ammonia synthesis converter to promote the reaction of these two gases to form ammonia. Typically, 15-30 percent of the hydrogen and nitrogen react as the gas mixture flows through the converter. The operating pressure in the ammonia converter generally is between 150 and 400 atmospheres, and the operating temperature of the catalyst is from about 400.degree. to about 550.degree. C., preferably about 500.degree. C.
The resulting gaseous reaction mixture leaving the converter is cooled in one or more stages. Most of the ammonia in the mixture condenses, forming a liquid product which is withdrawn from the sythesis system. Uncondensed ammonia, unreacted hydrogen and nitrogen, and other components of the gas mixture are recompressed and recyled back to the converter.
A stream of charge or make-up gas (H.sub.2 and N.sub.2) is added to the recycled gas before it enters the converter. As the system is to operate continuously, this make-up gas contains an amount of hydrogen and nitrogen which is equivalent to the product ammonia which is withdrawn plus any hydrogen, nitrogen and ammonia lost from the system.
Gases in the make-up stream other than hydrogen, nitrogen and ammonia may be divided into categories, i.e., catalyst poisons and inerts. The concentration of catalyst poisons, such as oxygen and oxygen compounds, must be reduced to a level at which they cannot exert a significant effect on the performance of the ammonia synthesis catalyst. Inert gases such as methane and argon can be tolerated in reasonably large quantities. However, the presence of such inert gases reduces the partial pressure of hydrogen and nitrogen in the synthesis loop (reactant charge system, converter, coolers, heat exchangers, separators, recycle compressor and recycle line), and thus reduces the amount of ammonia which is formed as the gas mixture passes over the catalyst.
All of the gases mentioned above have a limited solubility in liquid ammonia.
There are a number of feedstocks from which an ammonia charge and make-up gas can be produced, and a variety of methods which can be used to purify the crude make-up gas before it is piped to an ammonia converter. The following two examples serve to illustrate the range of processes employed industrially.
Currently, most ammonia is made from natural gas (essentially methane) or naphtha. After undergoing various reactions and purification steps, the feedstock is converted into a hydrogen-nitrogen gas mixture which, typically, contains from 0.8 to 1.3 percent by volumn of inert gases. As this gas mixture is recycled in a synthesis loop, its inert gas concentration typically is permitted to increase to from about 10 to about 20 percent.
The hydrogen produced in chlorine-caustic cells is also used to produce ammonia. The hydrogen-nitrogen mixture conveyed to the synthesis loop in such plants may contain from about 0.02 to about 0.3 percent inert gases, primarily argon.
Some of the inert gases, together with hydrogen and nitrogen, dissolve in the liquid product ammonia. This product ammonia is withdrawn from the high pressure synthesis loop and is piped to a let-down tank. This tank is maintained at a pressure, typically, of from about 200 to 300 psi. At this relatively low pressure, substantially all of the dissolved gases flash out of the liquid ammonia. However, a small amount of dissolved gas remains in the liquid leaving the let-down tank.
Even though the concentration of inert gases in the recycling gas stream of the synthesis loop is maintained at a fairly high level, as indicated previously, not all of the inert gases in the make-up gas stream dissolve in the ammonia product. Therefore, additional amounts of recycling gas must be bled continuously ("purged") from the system. This purge stream contains hydrogen, nitrogen and ammonia, in addition to the inert gases. Perhaps 5 percent by volume of the make-up gas is lost in this fashion, the exact amount being established by the process design.
It is important to recover the ammonia from the gases vented from the let-down tank and removed in the purge gas stream. Generally, this is done by cooling these gases in order to condense most of the ammonia. Alternatively, these gases can be scrubbed with water, or with any other liquid in which ammonia is soluble. Ammonia-free streams so obtained generally are burned in a furnace to recover their fuel values, released to the atmosphere, or conveyed to another process plant. In the past, the gases from the let-down tank have not been compressed and returned to the synthesis loop, because they still contain the unreactive inert gases and because it has been believed that the inert gas concentration in the circulating synthesis gas stream is already at the maximum desirable level.
It is possible to treat the purge gas stream and/or the gas from the let-down tank cryogenically, liquefying and removing substantially all of the inert gases. When this is done, the hydrogen and nitrogen can be returned to the ammonia synthesis loop. Alternatively, it is possible to treat the entire make-up stream cryogenically to remove substantially all of the inert gases before the stream is charged to the converter (e.g., Braun "Purifier" process).
When oil or coal is used as a feedstock, the resultant make-up gas can contain a high level of inert gases, for example up to about 1.5 percent by volume. In many instances, this make-up gas is scrubbed cryogenically ("Nitrogen Wash") to remove substantially all of the inert gases.
As described above, a relatively small portion of the hydrogen and nitrogen which is passed over the synthesis catalyst is converted into ammonia. Therefore, after removal of the ammonia, the unreacted hydrogen and nitrogen, together with any unreacted inert gases such as argon and methane, are compressed and again conveyed to the reactor. It is known that recycling of gases in the ammonia synthesis loop will result in an increase in the inert gas content of the synthesis loop. It is generally recognized that this recycling results in decreased conversion per pass of hydrogen and nitrogen for ammonia, and increased purging of reactant and inert gases from the synthesis loop. This is true in those instances in which the charge and make-up gas already contains a substantial inert gas content.
It is also known that gas from the let-down tank has been cooled to recover NH.sub.3 therefrom, and to discharge the remaining gas from the system because of the inerts therein.